Superoxo complexes, cyclic

Reduction potentials for /z-superoxo//z-peroxo-cobalt(III) couples have recently been obtained by cyclic voltammetry.739,740 Decomposition or dissociation of one or the other of the components of the couple, which frustrated measurements by other techniques, may be overcome by the use of fast scan rates. Protonation of the /r-peroxo group stabilizes the complex and EB values are pH dependent below pH 3. A similar stabilization on protonation of the peroxo bridge has been noted in the Ct2+-, V2+- and Eu2+-promoted reduction705 of the [(NHj)5Co(02)Co(NH3)5]4+ ion. The protonated species has Kh 10 dm3 mol-1 at 25 °C.705... 

One advantage of the cyclic voltammetry technique is that, to some extent at least, the lifetime of the experiment may be controlled through the scan rate, i.e. the rate at which the potential range is scanned and then reversed. This means that if one of the components of the couple is not very stable it may still be possible, by increasing the scan rate, to observe reversibility and hence determine °. Thus the n-superoxo/p-peroxo-dicobalt(III) couples in the complexes of the type [(NH3) Co()i-02)Co(NH3)5] + + can be determined readily even though the peroxo complex is not very stable and decomposes rapidly under conditions where the superoxo is stable. However, the scan rate must not be increased too much as at fast scan rates the rate of the electrochemical process becomes slow relative to the scan rate and irreversible behaviour (A p>57/n mV) will be obtained. In fact, this is the basis of a method for obtaining the heterogeneous rate constants of reversible couples. 

Reduction potentials determined by cyclic voltammetry have been reported for a number of superoxo and peroxocobalt complexes by [359]. 

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